The present invention generally relates to a thermal barrier coating for a gas turbine component, and more specifically to a thermal barrier coating having at least one ceramic layer having a segmented columnar microstructure and exhibiting increased stability. The present invention also relates to methods for coating a substrate with a thermal barrier coating having at least one segmented columnar ceramic layer.
In order to increase the efficiency of gas turbine engines, higher operating temperatures are now being used. However, the ability to operate at increasingly high gas temperatures within the engine is limited by the ability of the superalloy components to maintain their mechanical strength and integrity following continued exposure to the heat, oxidation, and corrosive effects of the impinging gas. It is known in the art to apply a protective thermal barrier coating to superalloy substrates to provide thermal insulation of the substrate as well as to inhibit erosion, and oxidation and hot gas corrosion of the substrate.
Prior art compliant thermal barrier coatings typically include an outer ceramic layer having a segmented columnar microstructure. Segmentation gaps between the individual columns of the outer ceramic layer imbue the characteristic of compliancy, and allow the columnar layer to expand and contract without developing stresses that may cause spalling. However, following exposure to temperatures in excess of about 2000° F., sintering of the multi-grained columns or columnar grains may close segmentation gaps between the columns. Once the gaps become closed, the ceramic layer can no longer accommodate the large ceramic-substrate thermal expansion mismatch, and the thermal barrier coating tends to spall leading to increased heat transfer into the component.
U.S. Pat. No. 6,203,927 B1 to Subramanian et al. discloses a thermal barrier coating having a sintering inhibiting material disposed on columns of a columnar ceramic layer. U.S. Pat. No. 6,296,945 B1 to Subramanian discloses a base columnar thermal barrier coating having a ceramic oxide sheath material thereon, wherein the sheath material is the product of a reaction between an applied precursor oxide material and material of the base columnar thermal barrier coating.
U.S. Pat. No. 6,103,386, to Strangman, discloses a thermal barrier coating having a columnar grained ceramic (e.g., zirconia) layer, and a sheath of bond inhibitor, such as alumina, disposed between the columnar grains, whereby the bond inhibitor imparts resistance to sintering of the columnar grains. However, the solubility of alumina in zirconia, or the melting of alumina and zirconia, limits the maximum temperature capability of such thermal barrier coatings to about 3000° F. In contrast, more advanced gas turbine engines require significantly higher surface temperature capability, namely up to about 3400° F.
As can be seen, there is a need for a thermal barrier coating having increased stability and integrity of columnar ceramic layers at higher operating temperatures. There is a further need for a thermal barrier coating which prevents the penetration or ingress of extraneous materials into segmentation gaps within the columnar ceramic layer(s). There is also a need for a stable thermal barrier coating which allows for the formation of effusion holes through the coating and the underlying substrate or component with no, or minimal, interfacial cracking or spalling of the thermal barrier coating.